A number of ultrasound imaging devices and systems have been proposed, and in many designs the problem exists of ensuring that each of the frames of an image acquired by the ultrasound device are properly registered with each other. Without such registration, there is the possibility that variance in mechanical movement of an ultrasound transducer, or inadvertent or purposeful movement of the device, will be unaccounted for in the image observed by the physician, resulting in undesirable changes in orientation or content of images.
For example, there are proposed devices in which an ultrasound transducer is turned (e.g. by a motor) around a longitudinal axis of the device (e.g. U.S. Pat. No. 8,214,010 to Courtney et al.). However, in some cases over time the positional error (i.e. the difference between the desired or intended position of the motor's rotor and its actual position relative to bodily tissue) can quickly grow to unacceptable levels. Even small velocity errors in the motor, when accumulated, can make an image appear to rotate on an ultrasound viewing screen or console, giving the impression that the imaging device is being physically rotated within the body even though it is stationary.
To address such a disconcerting or confusing and false effect, a rotary feedback mechanism may be used to precisely measure angular position so that each image frame acquired through ultrasound imaging is registered (i.e. oriented appropriately) with respect to previous frames. While a variety of rotary encoders are commercially available, such as hall effect sensors, incremental quadrature encoders, absolute gray encoders and potentiometers, such off-the-shelf devices are too bulky and complex to fit into an intravascular or other low profile device.
One solution is to place a wire or other structure within the acoustic window to use as a signal for registration. Yet this creates a defect in the acoustic window which causes intolerable problems with clear imaging and/or therapeutic applications. Accordingly, until now, designers were faced with choosing between registration or a clear viewing window.
There is thus a need for a registering device or system that is reduced in bulk and which will not block or otherwise cause intolerable defects in the acoustic window.